It has been discussed in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard that Discovery Reference Signals (DRS), also referred as discovery signal, could be used for Radio Resource Management (RRM) measurement as well as synchronisation in the topic of Licensed Assisted Access (LAA). LAA is sometimes also referred to as Licensed-Assisted Carrier Aggregation and concerns aggregation of a carrier wherein a primary cell is using a licensed spectrum to deliver critical information and guaranteed Quality of Service, and an unlicensed spectrum to opportunistically boost the transmitted data rate. The DRS detection is expected to be on one-shot basis.
In e.g. LTE Evolved UMTS Terrestrial Radio Access (E-UTRA), OFDM is used in the downlink; i.e. from a cell site to a User Equipment (UE). OFDM is a frequency-division multiplexing scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels.
For synchronisation purpose, the current synchronisation signal design for DRS with one length-62 Primary Synchronisation Signal (PSS) sequence and one length-62 Secondary Synchronisation Signal (SSS) sequence is not sufficient to support one-shot synchronisation in a low serving cell Signal-to-Noise Ratio (SNR) condition, e.g. SNR of −3 dB. Typically, the User Equipment (UE) according to previously known synchronisation signalling schemes need to detect the synchronisation signals successfully in multiple occasions in such a SNR condition. For DRS detection, the synchronisation signals shall satisfy one shot DRS measurement in a neighbour cell SNR condition, e.g. SNR of −6 dB. Therefore current DRS synchronisation signal design has to be enhanced in terms of performance.
Further, one key feature of LTE PSS is to enable low-complex receiver implementations which needs a symmetry property of the signal in the time domain.
In a first piece of prior art, the Licensed-Assisted Access (LAA) e-NodeB (eNB) transmits LAA DRS as in the form of Rel-12 DRS, which consists of one PSS sequence, one SSS sequence, antenna port 0 CRS and potentially Channel State Information Reference Signal (CSI-RS), if configured.
In a second piece of prior art, the LAA eNB repeats multiple synchronisation sequences in the frequency domain, e.g. the PSS and/or SSS sequence is repeated in the entire transmission bandwidth of the LAA system.
However, the disadvantages of the prior art can be summarised; firstly, DRS detection based on one shot is not ensured if a Long Term Evolution (LTE) Release 12 DRS is reused for LAA.
Secondly, it has not been shown how multiple synchronisation sequence shall be mapped in the frequency domain to achieve good detection performance as well as low complex receiver implementations.
The problem is how a transmitter such as an LAA eNB, transmits an OFDM symbol comprising multiple synchronisation sequences which provides good performance while also facilitates low complexity of the receiver.
It would thus be desired to provide a solution on how to map multiple synchronisation sequences to the OFDM symbol to achieve better synchronisation performance and low complex receiver implementations.